Preliminary studies concerning exposure of chromium to microsomes were completed at the end of 1997. In April - December 1998, systematic studies on microsomes treated with chromium were conducted once a week every week. Time course data from the ESR signal of chromium(V) was collected and compared to the UV-vis data for the reduction of chromium(VI). The rate of reduction of chromium(VI) and chromium(V) are very similar. Thus chromium is reduced in one electron steps and the existence of reactive intermediates of chromium is observed in human liver and lung microsomes. The goal of this research is to show that intracellular reductive metabolism of chromium(VI) to chromium(III), via reactive intermediates is thought to play a key role in the cytotoxicity, mutagenicity and carcinogenicity of chromium(VI) compounds. We have determined that certain aspects of rodent studies cannot be readily extrapolated to humans. This work is relevant when workers are exposed to chro mium . Exposure to chromium is associated with a wide array of toxic effects, including serious damage to internal organs and increased incidence of certain cancers.